ABSTRACT Tests were carried out to determine the holding strength of screws in the face and edges of plywood and oriented strandboard (OSB). Ten distinct proprietary boards were included in the study: three southern pine plywood, one Douglas-fir plywood, one hardwood plywood, and five OSB boards. Sheet metal type screws of sizes including 6AB, 8AB, 10AB, l2AB, and l4AB were used in the study. Initial tests were conducted to determine optimum pilot hole diameters. Predictive expressions were fitted to the results, which enable the withdrawal strengths of screws embedded in these materials to be predicted as a function of screw diameter and depth of penetration, and density of the board material. ********** Increasingly, manufacturers are using both plywood and oriented strandboard (OSB) in upholstered furniture frame construction. Reasons for this use vary, but elimination of several process variables associated with the use of solid wood are among the reasons most often cited. The rational product engineering of such frames in which screws are used as structural fasteners requires that designers have factual knowledge of the holding strength of screws in these materials. Such information must necessarily be gleaned from published test results or obtained from predictive expressions based on test results, when such expressions are available. A study was undertaken, accordingly, to obtain this information for representative plywood and OSB boards. Presumably, the information obtained for these boards could then be extrapolated to cover other similar boards offered to the furniture industry. Although the holding strength of screws in solid wood has been extensively studied and expressions have been developed to estimate holding strength as a function of either density (8) or shear strength parallel to the grain (3,4,5), the holding strength of screws in plywood and OSB has not been widely researched. One notable study of the withdrawal strength of screws from both the face and edge of Douglas-fir plywood was carried out by Johnson (7). In that study, 810 1-inch-long flat head screws were pulled from the faces and edges of 5-ply, Douglas-fir, interior A-D grade, 3/4-inch-thick plywood. This was a noteworthy paper, but both materials and screws have changed since it was published. A number of his observations and conclusions remain valid, however. Johnson stated, for example, that the resistance of screws pulled from the face of the plywood was greatest at machine speeds of 4 in./min., but that the results obtained at the other two speeds did not differ significantly. Similar results have been obtained with solid wood (6). Some guidance in this area is also provided by ASTM standards D 1761-77 and D 1037-78(1,2). Johnson (7) also noted that the withdrawal strengths of screws withdrawn from pilot holes that had the same size as the root diameter of the screws were significantly less than when the pilot holes were either 40 or 70 percent of the root diameter. Of particular importance, he also noted that no significant difference was found in strength values obtained with pilot holes that were either 40 or 70 percent of the root diameter of the screws. Page -1-
Transcript
ABSTRACT
Tests were carried out to determine the holding strength of screws in the face and edges of plywood and oriented strandboard (OSB). Ten distinct proprietary boards were included in the study: three southern pine plywood, one Douglas-fir plywood, one hardwood plywood, and five OSBboards. Sheet metal type screws of sizes including 6AB, 8AB, 10AB, l2AB, and l4AB were used in the study. Initial tests were conducted to determine optimum pilot hole diameters. Predictive expressions were fitted to the results, which enable the withdrawal strengths of screws embedded in these materials to be predicted as a function of screw diameter and depth of penetration, and density of the board material.
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Increasingly, manufacturers are using both plywood and oriented strandboard (OSB) in upholstered furniture frame construction. Reasons for this use vary, but elimination of several process variables associated with the use of solid wood are among the reasons most often cited. The rational product engineering of such frames in which screws are used as structural fasteners requires that designers have factual knowledge of the holding strength of screws in these materials. Such information must necessarily be gleaned from published test results or obtained from predictive expressions based on test results, when such expressions are available. A study was undertaken, accordingly, to obtain this information for representative plywood and OSB boards. Presumably, the information obtained for these boards could then be extrapolated to cover other similar boards offered to the furniture industry.
Although the holding strength of screws in solid wood has been extensively studied and expressions have been developed to estimate holding strength as a function of either density (8) or shear strength parallel to the grain (3,4,5), the holding strength of screws in plywood and OSB has not been widely researched. One notable study of the withdrawal strength of screws from both the face and edge of Douglas-fir plywood was carried out by Johnson (7). In that study, 810 1-inch-long flat head screws were pulled from the faces and edges of 5-ply, Douglas-fir, interior A-D grade, 3/4-inch-thick plywood. This was a noteworthy paper, but both materials and screws have changed since it was published. A number of his observations and conclusions remain valid, however.
Johnson stated, for example, that the resistance of screws pulled from the face of the plywood was greatest at machine speeds of 4 in./min., but that the results obtained at the other two speeds did not differ significantly. Similar results have been obtained with solid wood (6). Some guidance in this area is also provided by ASTM standards D 1761 -77 and D 1037-78(1,2).
Johnson (7) also noted that the withdrawal strengths of screws withdrawn from pilot holes that had the same size as the root diameter of the screws were significantly less than when the pilot holes were either 40 or 70 percent of the root diameter. Of particular importance, he also noted that no significant difference was found in strength values obtained with pilot holes that were either 40 or 70 percent of the root diameter of the screws.
OBJECTIVES
The primary objective of the study was to develop estimates of both face and edge screw holding strength that could be used in the product engineering of furniture frames constructed of plywood or OSB. Secondary objectives included determination of the effect of pilot hole size on withdrawal strength and the relationship of withdrawal strength to screw diameter and depth of penetration. An important objective was the development of predictive expressions that could be used to estimate holding strength as a function of board properties, screw diameter, and depth of penetration.
DESCRIPTION OF MATERIALS
The boards included in the study are described in Table 1. A coding system was used to identify the boards used in this study. OSB stands for oriented strandboard; SPLY stands for southern pine plywood; HPLY stands for hardwood plywood; DFP stands for Douglas-fir plywood. The boards were obtained from several different suppliers.
In the case of the 5-ply plywood construction (SPLY-1, SPLY-3, and DFP-5/8), the center ply was aligned parallel to the face plies. Thus, 3 plies were aligned parallel to the grain direction of the face plies and 2 plies perpendicular to the face. In the case of the 6-ply plywood construction (SPLY-2 and HPLY), the 2 center plies were aligned in the same direction; as a result, a total of 4 plies were aligned parallel to the face plies and 2 plies perpendicular to the face. In the case of 7 -ply construction (DFP-3/4), 4 plies were aligned
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parallel to the face and 3 plies were perpendicular to the face. In the case of 4-ply plywood construction (DFP-3/8 and DFP-1/2), the 2 center plies were aligned in the same direction that is perpendicular to the long dimension of the panel while the 2 face plies were aligned in the same direction that is parallel to the long dimension of the panel.
All of the boards were kept in an environmentally controlled testing room set to produce an average of 7 percent equilibrium moisture content. Representative 48 - by 24-inch sections cut from the boards were measured and weighed in order to determine the density of the boards.
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DESCRIPTION OF TESTS AND CONSTRUCTION OF SPECIMENS
PILOT HOLE TESTS
Pilot hole studies were conducted in order to obtain estimates of the holding strengths of screws, both when pilot holes were not used (face withdrawal only) and when pilot holes of optimum size were used. Only two board types were used in this study: SPLY-3 and OSB-1. Screw sizes included 6AB, 8AB, 10AB, 12AB, and 14AB. Four pilot hole diameters were used with each screw size. Each pilot hole differed from the preceding one by 1/64 inch. Also, one set of face withdrawal specimens was constructed without pilot holes. A previous study conducted with hardwoods indicated that holding strength decreased rapidly once the pilot hole exceeded the root diameter of the screw. An attempt was made, accordingly, to set the largest pilot hole diameter equal to or slightly larger than the root diameter of the screw. The three remaining pilot holes were then drilled 1/64, 2/64, and 3/64 inches smaller than the largest pilot hole.
The configurations of the specimens used in the tests are shown in Figure 1. All of the specimens used in the tests measured 6 inches square. In the case of the face withdrawal specimens, the screws protruded from the specimens, whereas the screws were embedded 1 inch in the edge withdrawal specimens. A 1/2-inch hole was drilled through the broad face of each edge withdrawal specimen at its center in order to provide a point of attachment for the testing machine jigs. Three replicas of each specimen combination were constructed.
Four pilot holes were drilled for a single screw size in each specimen. This procedure was followed in order to reduce variability and thereby provide a better indication of withdrawal strength versus screw diameter. In the edge withdrawal specimens, pilot holes were drilled at the center of the edge. Also, no edge specimens were prepared without pilot holes since the screws tended to cause the specimens to delaminate.
SCREW WITHDRAWAL STRENGTH TESTS
The objective of the screw withdrawal strength tests was to determine the holding strength of various size screws penetrated to various depths in the edges of plywood and to 1 inch in OSB. These tests were conducted in essentially the same manner as the pilot hole tests with the following exceptions or additions.
1. The face tests were conducted with OSB-1 through OSB-5, SPLY-1 through SPLY-3, and HPLY
2. Optimum pilot holes were used in keeping with the findings of the pilot hole tests.
3. In the case of the face withdrawal specimens, one set of specimens was constructed in which the screw was embedded to the full thickness of the specimen (Fig. 2a); a second set of specimens was constructed in which the full tip of the screw protruded from the specimen (Fig. 2b). Four replicas were prepared for each combination.
Locations of the screws in the specimens are shown in Figure 1. In practice, only one screw was inserted at a time. Once it had been withdrawn, the next screw was inserted in the second position and the test was repeated. This procedure was repeated until a complete set of five screws had been tested.
Two sets of edge withdrawal specimens were constructed and tested. The first set was constructed with SPLY-1, SPLY-2, SPLY-3, HPLY, DFP-3/4, and OSB-1 through OSB-5. All screws were embedded 1 inch in the edges of the specimens. The primary purpose of this set of tests was to determine the relationship of withdrawal strength to screw diameter.
The second set of specimens was constructed with DFP-3/8, DFP-1/2, DFP-5/8, and DFP-3/4 (OSB was not included). Depths of screw penetration included 1/2, 3/4, 1, and 1-1/4 inches. The primary purpose of this set of tests was to determine the relationship of withdrawal strength to depth of penetration in plywood. Only one diameter screw was used in each specimen. Screws were inserted perpendicular to the edge of the specimen at the midpoint of each edge. Thus, two screws were tested along the grain (parallel to the 96 -inch length of the panel) and two were tested perpendicular to the grain (along the 48-inch width of the panel).
GENERAL METHOD OF TEST
All of the tests were carried out on a Riehle universal testing machine. Rate of loading was 0.1 in./min. Ultimate load was taken as the holding strength (withdrawal strength) of the screw. The fixture used to hold the screw head is shown in Figure 3. A different size of fixture was used with each size screw in order to provide maximum support to the underside of the screw heads. The screw holder itself was coupled to a length of rod that passed through the upper crosshead and ball seat of the testing machine. It was anchored in place in the crosshead with a shaped nut so that it was centered in the ball seat. This fixture was used in both the edge and face withdrawal tests.
RESULTS
PILOT HOLE TESTS
Results of the pilot hole tests are given in Table 2. Results are also shown in Table 3 expressed as percentages of root diameters. Due to the composite nature of the boards, highest strength yielding pilot hole sizes varied as a percentage of root diameters among the screw sizes as well as between two types of boards. As can be seen, the average optimum pilot hole expressed as a percentage of the root diameter of the screw amounted to 64 percent in the face of plywood and 71 percent in the face of OSB.
Similarly, the average optimum pilot hole expressed as a percentage of the root diameter of the screw amounted to 80 percent in the edge of plywood and 82 percent in the edge of OSB.
FACE AND EDGE SCREW HOLDING TESTS
Numerical results for the face and edge holding strength of the screws in the face and edge of plywood and OSB are given in Table 4. Results for the holding strength of screws embeded 1/2, 3/4, 1, and 1-1/4 inches in the edge of Douglas-fir plywood are given in Table 5.
DISCUSSION
Given the variability of the results obtained, an attempt was made to derive simple expressions to represent the data. The form of these expressions was based on previous studies (4,5) carried out with solid wood to estimate holding strength of screws as a function of depth of penetration, screw diameter, and shear strength parallel to the grain. In general, therefore, an attempt was made to develop equations based on linear relationships provided they did not greatly degrade the quality of the predictions. Accordingly, an expression of the following form was fitted by means of non-linear regression techniques to the results for face withdrawal and for edge withdrawal from both plywood and OSB:
y = a[D.sup.b] [(L - cD).sup.d][W.sup.e]
In this expression, y = screw holding strength (lb.), a, b, c, d, and e = regression coefficients; D = screw diameter (in.), L = depth of penetration (in.), and W = density (pcf). The term [(L - cD).sup.d] is used to take into account both the depth of penetration of the screw, represented by the term L, and the loss in strength that occurs because the tip of the screw is not in contact with the composite when pilot holes are used (hereafter called "tip effect") represented by the term cD. It should be noted that the tip is also not fully effective when pilot holes are not used. This is a particularly important consideration when estimating the strength of short screws. It should be noted that the term D refers to the major or outside diameter of the screw. Major screw diameters may be calculated by means of the following expression:
D = 0.06 + 0.013N
where D = major diameter of the screw (in.); N = gage of the screw (e.g., a #10 screw has a major diameter of 0.190 inches).
The expressions were fitted to the results by means of statistical non-linear regression techniques and then simplified, if possible, in accordance with the outcomes of the analyses. Results of the statistical analyses (including the expression developed, the accompanying coefficient of determination value, [r.sup.2], the maximum and minimum deviations between predicted and observed values, and the standard deviations) are given in Table 6.
Since all of the plywood except Douglas-fir measured a nominal 3/4 inch thick, only the face withdrawal data for screws in Douglas-fir plywood was used to determine the relationship of depth of penetration to holding strength. As previously discussed, the term [(L - cD).sup.d] was used to take tip effect into account. The best fit to the data was obtained with the factor [(L - 2D/3).sup.1.4] with [r.sup.2] = .905. When this factor was simplified, however, with d = 1.0, the factor gave the best results with [r.sup.2] = .856. This term is incorporated into the simplified expression given in Table 6, i.e., y = [9D.sup.0.5](L - D)[W.sup.1.5]. With OSB, on the other hand, best results were obtained with the factor (L - 2D/3) in the simplified expression y = 0.87[D.sup.0.5](L - 2D/3)[W.sup.2] as is also shown in Table 6.
In the case of edge withdrawal in plywood, the best fit was obtained with the factor [(L - D).sup.0.78] where [r.sup.2] = 0.670. With d = 1.0, so that the term (L - D) is obtained, the [r.sup.2] value is slightly reduced to 0.630. This term is incorporated into the simplified expression given in Table 6, i.e., y = 6.8[D.sup.0.5](L - D)[W.sup.1.5].
Only one depth of penetration, 1 inch, was used in the edge holding tests of OSB. As a result, no depth of penetration term is given in the predictive expression for edge withdrawal from OSB in Table 6. Likewise, no correction term is given for tip effect. Thus, the following simplified expression results y = 0.66[D.sup.0.5][W.sup.2] (also given in Table 6).
In general, non-linear regression analyses of the test results indicated that relatively simple power expressions could be used to estimate the holding strengths of screws in the face and edge of plywood and OSB. Typically, these expressions had coefficients of determination that ranged from 0.571 to 0.779. Expressions under -predicted holding strengths from a low of 38 to a high of 108 percent and over-predicted strengths from a low of 36 to a high of 69 percent. The standard deviation of the differences between predicted and observed values expressed as a percentage of predicted values ranged from a low of 16 to a high of 27 percent.
In general, therefore, the withdrawal strength of screws in composites must be regarded as somewhat variable, but the statistics presented in the paper allow this variation to be taken into account in the design of a screw-based joint.
CONCLUSIONS
Results of the tests indicate that the holding strength of screws in both the face and edge of plywood and OSB may vary considerably from board to board and also within boards. This variability is likely more closely related to process variables than basic wood properties since boards manufactured from the same species may still exhibit significantly different holding strengths. In general, therefore, these results indicate that predictive expressions should be based on results derived from a larger population of boards and should include pertinent process variables. The problem that exists for the practicing furniture engineer, however, is that such processing information is not readily available, and if available, would not likely be specific to the boards he is using at a given time.
Final 49 19Oriented strandboard: OSB-l, OSB-2,OSB-3, OSB-4, OSB-5Initial
Final 36 16
Edge withdrawal
Plywood: SPLY-l, SPLY-2, SPLY-3, HPLY Intial
Final 69 27Oriented strandboard: OSB-l, OSB-2, OSB-3, OSB-4, OSB-5 Initial Final 39 20
(a)Under: maximum under-predicted by percentage.
(b)Over: maximum over-predicted by percentage.
(c)Standard deviation of the under-and over-predicted values.
LITERATURE CITED
(1.) American Society for Testing and Materials. 1984. Evaluating the properties of wood-base fiber and particle panel materials. Standard D 1037-78. ASTM, West Conshohocken, PA.
(2.) _____. 1984. Mechanical fasteners in wood, Standard D 1761-77. ASTM, West Conshohocken, PA.
(3.) Eckelman, C.A. 1973. Holding strength of screws in wood and wood-based materials. Res. Bull. 895. Purdue University Agri. Expt. Sta., West Lafayette, IN. 15 pp.
(4.) _____. 1975. Screw-holding performance in hardwoods and particleboard. Forest Prod. J. 25(6):30-35.
(5.) _____. 1978. Predicting withdrawal strength of sheet metal type screws in selected hardwoods. Forest Prod. J. 28(8):25-28.
(6.) Johnson, J.W. 1959. Screw-holding ability of western woods: Effects of test variables. Special Tech. Pub. 282. Am. Soc. for Testing and Materials, West Conshohocken, PA.
(7.) _____. 1967. Screw-holding ability of particleboard and plywood. Rept. T-22. Forest Research Lab., Oregon State Univ., Corvallis, OR.
(8.) USDA Forest Service. 1974. Wood Handbook: Wood as an Engineering Material. Agri. Handb. 72. U.S. Gov. Printing Office, Washington, DC.
YUSUF Z. ERDIL *
JILEI ZHANG *
CARL A. ECKELMAN *
* Forest Products Society Member.
The authors are, respectively, Graduate Research Assistant, Dept. of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907; Assistant Professor, Forest Products Laboratory, Mississippi State University, Mississippi State, MS 39762-9820; and Professor, Dept. of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907. Approved for publication as Journal Article No. FP 205 of the Forest and Wildlife Research Center, Mississippi State University. This paper was received for publication in March 2001. Reprint No. 9277.
